Optical attenuation coefficients and liquid water content relationship in fog, at seventy-four wavelengths from 0.35 to 90 μan

Abstract

The relationships between the optical volume scattering, absorption and extinction coefficients ( σ s , σ a , σ e ), and the fog liquid water content W, have been analysed at 74 wavelengths A from 0.35 to 90 μm. Computations have been carried out on the basis of 239 experimental droplet size spectra of valley, advection and radiation fogs; they mainly concern the determination of the linear correlation coefficients ( R s , R a , R e ) between σ s and W ks , σ a and W k a and σ e and W k e and of the means of the absolute values of relative deviations ( μ s , μ a , μ e ) of σ s , σ a , σ e from the corresponding values of best fit. The results obtained when considering all the fog types together are as follows: 1. (1) a good relationship between σ s and W k s exists in the 10–14 μm λ-region where R s lies in the 0.973–0.994 interval and μ s is at the most 18 %; in the 14–40 μm λ-region the relationship can be still regarded as reliable, with 0.963 ⩽ R s ⩽ 0.987 and μ s ⩽ 19%. 2. (2) The relationship between σ a and W ka is good in the 0.35–2.4, 3.5–5.8, 6.7–11 μm λ-regions and for λ ⩾ 40μm; for A = 6.7–11 μm, 0.971 ⩽ R a ⩽ 0.988 and μ a ⩽ 16%. 3. (3) The σ e - W k e relationship is good in the 10–12 μm λ-region and for λ ⩾ 30 μm; for λ = 10–12 λm, 0.971 ⩽ R e ⩽ 0.989 and μ e ⩽ 16%. 4. (4) Around λ = 11.5 μm the σ s - W k s relationship becomes partially independent of the. droplet size distribution; the same holds for σ a - W k a around λ = 1, 3.8, 5.3, 9 μm and for σ e - W k e around λ = 11 μm. 5. (5) The behaviour of the pairs of parameters ( R s , μ s ), ( R a , μ a ), ( R e , μ e ) with the fog type shows that on average the valley fogs behave best and advection fogs worst.